The recent proliferation of Wi-Fi access points in wireless local area networks (WLANs) has made it possible for navigation systems to use these access points for position determination, especially in areas where there are a large concentration of active Wi-Fi access points (e.g., urban cores, shopping centers, office buildings, and so on). For example, a client device or station (STA) such as a cell phone or tablet computer can use the round trip time (RTT) of signals transmitted to and from the access points (APs) to calculate the distances between the STA and the APs. Once the distances between the STA and three APs are calculated, the location of the STA can be estimated using trilateration techniques. The distances between APs can also be calculated using the RTT of signals transmitted between the APs, and thereafter trilateration techniques may be used to determine and/or verify the locations of the APs. Similarly, the distance between a pair of STAs can also be calculated using the RTT of signals transmitted between the STAs, and thereafter trilateration techniques may be used to determine and/or verify the locations of the STAs.
For example, referring to FIG. 1, the distance (d) between an access point (AP) and a mobile communication device (MCD) can be calculated as d=c*tp/2=c*(RTT−TAT)/2, where c is the speed of light, tp is the summation of the actual signal propagation times of request (REQ) and acknowledgement (ACK) signals exchanged between AP and MCD, and TAT is the turn-around time (or processing delay) corresponding to the time delay between MCD receiving REQ from AP and beginning its transmission of ACK to back AP. Typically, the value of RTT is measured by AP as the difference between the time of departure (TOD) of REQ from AP and the time of arrival (TOA) of ACK at AP, and the value of TAT is estimated by AP.
Unfortunately, different make-and-models (and sometimes even same make-and-models) of mobile communication devices (as well as wireless access points) have different processing delays, and therefore it is difficult for AP to accurately estimate the correct TAT for MCD. Further complicating matters, the TAT for MCD may vary between packet transmissions (e.g., the TAT of MCD may not be constant over a given period of time). Thus, because of the large value of c with respect to d, errors in the calculated value of RTT resulting from slight errors in the estimated value of the TAT of MCD can lead to large errors in the calculated distance d between AP and MCD. In addition, when AP receives the ACK from MCD, the measured TOA value may be different from the actual TOA because of multipath effects, internal timing errors, and/or measuring errors, which can also lead to errors in the calculated distance d between AP and MCD.
Thus, there is a need to increase the accuracy of RTT ranging operations performed between two wireless communication devices.